Improvements in photolithography over the years have been responsible for many of the advances in integrated circuit technology in general. As these circuits have grown increasingly more complex, the number of different layers in the finished product has also grown considerably. Although it is general practice to planarize the surface of the wafer at regular intervals throughout the process, it is nevertheless impossible to avoid situations in which the surface onto which a given image is being focused for photolithographic purposes is quite uneven. It thus becomes necessary to devise a technique for always obtaining the "best focus", i.e. the best compromise for that particular surface.
Solutions to this "best focus" problem are included in commercial steppers such as the PAS-5000/50 I-line system which uses averaged information of step height over a fixed probing area of an image field to determine the focal plane and to perform field by field focusing. The focusing system comprises a focus sensor, control electronics, and actuator. It is illustrated in FIG. 1. The focus sensor 11 operates by reflecting laser light off the surface of wafer 12 (which is free to move vertically relative to projection lens 15) into cat's eye reflector 13 which sends the beam back along the same path. In effect, the sensor determines when the diameter of light spot 14, on the wafer surface, is at a minimum. Projection lens 15 is positioned to be directly above spot 14 and is both vertically and horizontally fixed relative to the focusing system in such a way that when spot 14 is at minimum diameter an image projected through the lens onto the wafer surface will be in focus there.
FIG. 2 is a schematic representation of semiconductor wafer 12, showing a small array of fields (or reserved areas) such as 22 within which various images needed to process the integrated circuit will be formed. In a real wafer, the number of such `integrated circuit` fields will normally be much larger (generally between about 90 and 150 such fields per wafer). Also seen in FIG. 2 is light spot 14 (as originally shown in FIG. 1). Because of the geometrical constraints associated with this prior art design, the light spot 14 is necessarily located fully or partly within an integrated circuit field.
In the course of using steppers of this type, we have determined that the focus spot plays such a great role in the focusing system that any minor variation in its position, size and overlap has a major effect on focusing performance. The "best focus" is usually determined as the center between two defocus points, where critical dimensions (CDs), resist side-wall profile, and resist loss would lead to final CD (usually .+-.4% variation) and profile errors after etch.
Experiments demonstrated that the step topography was an important factor in deciding the best focus. Due to its dependence on step topography, the collected information of the probing area, where the focus laser spot was hit, could be strongly affected by local variations in layout and topography. The auto-focus design of PAS-5000/50 obviously detected the surface substrates already patterned, and, inevitably introduced focus errors. Some of these can be enormous particularly when the laser spot position has a displacement on the wafer (say 800 microns) with respect to the original position. The reason lies in the different step topography information that is collected by the probing laser spot with fixed detection area.
The problems described above are most severe during processing of back-end lithography layers when lack of surface smoothness is most prevalent. Among the most common defects that are introduced as a result of this are blind via holes and damaged metal lines. There is therefore a need for an automatic focusing process that is not dependent on, nor subject to error when dealing with, varying topographies.
A routine search of the prior art was conducted but no references describing the process taught by the present invention were encountered. Several references of interest were, however, found. For example, Farino et al. In U.S. Pat. No. 5,783,340, show an auto-focusing alignment technique. Larsen (U.S. Pat. No. 4,580,900) teaches an auto-focus alignment and measurement system that uses a photodiode while Allen, in U.S. Pat. No. 4,615,621, teaches a similar system.